1. Field of the Invention
The present invention relates to a concrete admixture which is added to a concrete. More specifically, it relates to a concrete admixture which is obtained by mixing a silicate with an acid in the presence of water and then solidifying the mixture, a process for producing the same, a concrete using the concrete admixture, a process for constructing a concrete structure using the concrete admixture, and a process for producing a concrete product using the concrete admixture. Further, it relates to a mortar admixture which is added to a mortar, a process for producing the same, a mortar using the mortal admixture, and a process for producing a mortar product using the mortal admixture. Still further, the invention relates to blended cement.
2. Description of the Related Art
Since a concrete is high in compressive strength, easy to mold and less costly, it has so far found wide acceptance in the construction field. In recent years, since structures in particular are large-sized and diversified, many studies have been made for more enhancing a compressive strength of a concrete. In order to increase a strength of a concrete, for example, a method using various admixtures has been reported. This is the same with a mortar.
JP-A-61-155237 (U.S. Pat. No. 4,892,586) discloses a method for producing a dam concrete characterized in that an organic acid and a set accelerating inorganic salt are contained therein. It describes that a strength can be increased by containing an organic acid and a set accelerating inorganic salt without the increase in exothermic heat, and that the method is suited for placing a large amount of a concrete such as a dam concrete. Examples of the organic acid used here include various carboxylic acids such as citric acid and fumaric acid, and examples of the set accelerating inorganic salt include alkali metal salts such as sodium carbonate and sodium silicate. In Examples of this document, the improvement in compressive strength is specifically confirmed.
JP-A-2001-294461 describes a concrete modifier containing water glass and a polycarboxylic acid or derivatives thereof. This concrete modifier is coated on a surface of a concrete after cured in solution. It is described that deterioration due to neutralization of a concrete or occurrence of cracks can thereby be prevented or suppressed.
The most important mechanical characteristics of a concrete are that a compressive strength is high while a tensile strength, a shear strength and a flexural strength are low. In general, although a compressive strength of a concrete can be increased, a tensile strength of a concrete is limited. Thus, it is difficult to increase a tensile strength beyond the limit. Accordingly, in a site where a concrete structure is constructed, occurrence of cracks owing to an insufficient tensile strength is a serious problem.
Since a concrete is low in tensile strength in comparison to a compressive strength, the strength designing is performed using separately a reinforcement against a tensile force upon neglecting a tensile strength of a concrete for securing a safety. As the reinforcement, a reinforcing steel and a prestressing steel are listed. A strength in a whole structure is actually secured upon using these members. However, since structures are large-sized and diversified as stated above, an amount of steel used for securing the strength of the concrete structures is increased, and excessive bar arrangement is thus unavoidable. In this case, a fluid concrete is not satisfactorily charged when placed in a form, or a work of bar arrangement is itself intricate, which leads to the increase in costs.
A concrete of which the temperature is increased by heat generation owing to a hydration reaction after placing is shrunk in cooling. When both ends of a concrete are confined in shrinking, a tensile stress is generated, whereby occurrence of cracks is often induced. It is thus important to increase a tensile strength of a concrete within a relatively short period of ageing time during which a hydration reaction proceeds. Further, a concrete immediately after placing is wet. However, when a concrete is exposed to air, it is shrunk by drying. To cope with shrinkage by long-term drying, it is important to increase a tensile strength of a concrete.
A method in which a short fiber reinforcement made of inorganic fibers such as asbestos and glass fibers or synthetic fibers is used to improve a tensile strength of a concrete has been known. In this method, however, a fluidity might be impaired, or a dispersibility of a reinforcement might be insufficient to form clumps, which leads to the increase in costs. Accordingly, this method is used only in limited applications.
In the method described in JP-A-61-155237, a strength is increased by adding an admixture to a concrete. However, only a compressive strength is increased, and there is no description on a tensile strength. In the method described in JP-A-2001-294461, the modifier is coated on the surface of the cured concrete, and the deterioration of the concrete such as occurrence of cracks can be prevented, but a tensile strength is not positively increased.